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We have subsonic and supersonic for speeds below or above the speed of sound. What is the equivalent for the speed of light?

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Superluminal or FTL for the positive, but nothing special for ordinary velocities, even relativistic ones. Sublight or sub-lightspeed are some of the terms that appear in science fiction stories, but they're guessing at future usages rather than reporting them. Since nothing moves faster than light that we know of, there's little non-SF usage. –  John Lawler Mar 15 at 18:12
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Sci-fi writers sometimes use FTL for "Faster Than Light", but obviously so far as the boffins are concerned no such thing is possible anyway. And we don't really need "sub-light speed" since that's basically everything except light. But there are several thousand written usages to show that people do use it. –  FumbleFingers Mar 15 at 18:14
    
@FumbleFingers I was just typing the same about the lack of necessity for sub-lightspeed. –  David M Mar 15 at 18:14
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@David: Oh, my sainted aunt! Who would have to buy the Cokes in a three-way Jinx? –  FumbleFingers Mar 15 at 18:19
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4 Answers 4

up vote 18 down vote accepted

I've seen superluminal and subluminal. Sometimes sub-lightspeed

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I've definitely seen sub-lightspeed. –  David M Mar 15 at 18:14
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I would point out that the linked Wikipedia article, "Superluminal motion", concerns a phenomenon where something merely appears to be moving faster than light despite not actually doing so. Nevertheless, physicists do use "superluminal" and "subluminal" to refer to true faster- and slower-than-light motion respectively, and most physicists would recognize these words as the closest equivalents to "supersonic" and "subsonic". (The phenomenon in the article probably shouldn't be called superluminal motion at all; it's just a misleading name we're stuck with due to tradition.) –  David Z Mar 15 at 21:31

Particularly if you are referring to particles, you could use tachyonic for faster-than-light and bradyonic for slower-than-light.

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Let's not leave this one out: tardyonic. –  Jim Mar 15 at 18:32
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@Jim That sounds like you are late for music class . . . –  David M Mar 15 at 18:42
    
"bradyonic" ..and that makes me think of the Brady Bunch. –  Andrew Thompson Mar 16 at 3:01
    
@Jim: Presumably tardyonic refers to the speed of a TARDIS? ;-) –  R.. Mar 16 at 16:39
    
Speed has no meaning to a time lord. –  Jim Mar 16 at 16:43

For speeds below the speed of light, you can consider the word sublight http://stargate.wikia.com/wiki/Sublight_engine

For speeds above the speed of light, you can consider F.T.L., meaning faster-than-light. See here

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Are either of those used outside of science fiction? –  IQAndreas Mar 15 at 21:06
    
@IQAndreas They are used colloquially among physicists. –  David Z Mar 15 at 21:22
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any discussion of speeds equal to or greater than c is by definition science fiction. –  DougM Mar 15 at 22:13
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@DougM There are plenty of ideas involving FTL event propagation which are more than science fiction even if not established fact either. Tachyons, wormholes, and Alcubierre drive are all more than science fiction. Even if none of them turn out to be true, or at least practical for communication/travel, they are still reasonable hypothesis to be considered given our current understanding. The idea of FTL travel/communication is a useful one to have, even if it really is impossible. –  smithkm Mar 16 at 3:10
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@DougM Equal to or greater? The speed c appears quite frequently in works of science fact. :) Some things (e.g., light in vacuum) even go at that speed. –  Eliah Kagan Mar 16 at 4:12

Faster than light has to be Warp speed. Everything else is just normal, impulse power.

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Notwithstanding the existence of some real-world theoretical analogues, warp speed is a fictional technology. Nor is "everything else" impulse power, unless you are generously inviting real technologies like solar sails under that umbrella. NASA Sunjammer, launching later this year! –  Jonathan Van Matre Mar 16 at 7:52
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This might be a reasonable answer on Science Fiction & Fantasy.SE: on ELU it is unhelpful. –  TimLymington Mar 16 at 19:00

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